Sheik vs. Mario is a matchup I've been trying to heavily analyze this past while. At first it seemed to me like it was one of the few matchups that are (slightly) not in Sheik's favour, but now I think I might reconsider that after adapting and really figuring out how to deal with Mario.
Let me break down Mario's best options for you:
Neutral Air - this is a move Mario will use to break out of Sheik's bread and butter combos and strings; be very aware of it. It comes out frame 3 and is very fast and lasts a while. This is the sole reason Sheik should be resetting her combos and not going for long strings. It is also a very safe way for Mario to approach, provided he's throwing fireballs while doing it.
Down Air - A good option for Mario to land, could be a mix up from the usual nair.
Fireballs - Probably the most annoying thing Mario has and I'll talk about this in a bit.
Down Smash - Extremely good; Mario will go for it if they predict a roll or not sure what to do, little lag and covers lots of space, in addition to being powerful. He might use this in ledges predicting a normal get up or a roll; be aware of it.
Up Smash - EXTREMELY GOOD. This move is probably Mario's best move and up there as one of the best smash attacks in the game. Be EXTREMELY aware of it and this is pretty much the only reason Sheik should NOT be approaching from the air, which I'll cover in a bit.
Forward Smash - Not really threatening because its slow and has little range, but it's powerful.
With that being said, Sheik's neutral game in this matchup is very even. It's better if you stay back and play the needles game; charge needles whenever there is a chance, and throw them whenever you can (beware of Cape, it can be annoying, but if you read it you can shoot them afterwards, there's plenty of lag on the cape). Don't mindlessly approach, in fact, I'd advise to not approach at all unless you know it's safe. Mario can do two things here: play the camping game like you are and shoot fireballs; this isn't so bad, Sheik will win because she has the superior projectile, bigger range, and fireballs can easily be blocked anyway, but it could be slightly annoying. The other option is Mario approaching; this is what you want, to force an approach. If they approach you can try going in then, but beware of nair and always reset your combos by either pulling back and charging needles or getting a grab or something.
When they approach, they will always be looking for a grab (or with nair as I said before) because that's where Mario excels at, and especially at lower percents where they're hungry for the 50% combo from dthrow -> utilt -> uair shenanigans (which is easily escapable if you block/roll at very low percents or double jump at higher percents).
Usmash is the sole reason Sheik should never be approaching from the air. It's an extremely safe option for Mario to go for and it covers all the space around Mario (up, in front, and behind), has good range, and is an incredible kill move. This is mostly useless information but Sheik can actually duck and not get hit but only in front of Mario, she'll get hit if she's behind him. Don't challenge Mario from the air, but you can challenge him while he's in the air because Sheik's fair or nair outranges Mario's nair, which is good to keep in mind.
Don't try to go for unsafe things such as dthrow to Vanish trap which you can get punished for, or bouncing fish reads on the ground which will leave you with a lot of lag and a big opportunity for the opponent to punish you. Try to gimp Mario with vertical needles, nair, or the way you'd normally gimp with Sheik anyway.
Tomahawk grabs (faking an approach from the air to bait shield, then quickly dropping and grabbing them) is a good option provided you're grabbing them for the back so that they can't quickly ftilt, dtilt or anything like that, so there's that. Sheik's bair is also very strong in this matchup because it's a really strong move with a lot of range, so you might wanna practice your RAR skills.
tl;dr:
Camp with needles. Never approach from the air because of Usmash. Beware of Nair and always reset your combos and strings. Play extra campy and never approach unless its safe, this matchup demands it.
Luigi is the same btw, except in that matchup you don't have to be aware of the cape and fireballs are a lot less annoying because of their short range. Mario and Luigi are two characters that I found force me to change my playstyle from being mostly aggressive to very campy and defensive, only taking the safe options.
If you have any questions do ask me and hopefully I'll have an answer. I really hope this helped you, and sorry if it didn't :o
I would post a video or two of me playing against a rather decent Mario as an example of what I'm saying but I kinda don't have access to my WiiU and I'm a bit lazy .-.
